1. Field of the Invention
This invention generally relates to optical communication systems and more particularly to an optical receiver.
2. Description of the Related Art
The telecommunications network serving the United States and the rest of the world is presently evolving from analog to digital transmission with ever increasing bandwidth requirements. Fiber optic cable has proved to be a valuable tool, replacing copper cable in nearly every application from large trunks to subscriber distribution plants. Fiber optic cable is capable of carrying much more information than copper with lower attenuation.
The T-1 standards committee of ANSI has provided a draft document, xe2x80x9cANSI T1.105-1988xe2x80x9d, dated Mar. 10, 1988, which sets forth specifications for rate and format of signals that are to be used in optical interfaces. The provided specifications detail the Synchronous Optical Network (SONET) standard. SONET defines a hierarchy of multiplexing levels and standard protocols which allow efficient use of the wide bandwidth of fiber optic cable, while providing a means to merge lower level DS0 and DS1 signals into a common medium. In essence, SONET established a uniform standardization transmission and signaling scheme, which provided a synchronous transmission format that is compatible with all current and anticipated signal hierarchies. Because of the nature of fiber optics, expansion of bandwidth is easily accomplished.
Currently this expansion of bandwidth is being accomplished by what is known as xe2x80x9cwavelength division multiplexingxe2x80x9d (WDM), in which separate subscriber/data sessions may be handled concurrently on a single optic fiber by means of modulation of each of those subscriber datastreams on different portions of the light spectrum. WDM is therefore the optical equivalent of frequency division multiplexing (FDM). Current implementations of WDM involve as many as 128 semiconductor lasers each lasing at a specific center frequency within the range of 1525-1575 nm. Each subscriber datastream is optically modulated onto the output beam of a corresponding semiconductor laser. The modulated information from each of the semiconductor lasers is combined onto a single optic fiber for transmission. As this digital signal is passed across a SONET network, it will be subject at various intervals to amplification by, for example, Erbium doped amplifiers and dispersion compensation by, for example, optical circulators with coupled Bragg filters. At each node in the network, e.g. central office or remote terminal, optical transceivers mounted on fiber line cards are provided. On the transmit side, a framer permits SONET framing, pointer generation and scrambling for transmission of data from a bank of lasers and associated drivers, with each laser radiating at a different wavelength. On the receive side, the incoming signals are separated into channels detected by photodetectors, framed and decoded.
As more and more optical signal equipment (transmitting, receiving, amplification, coherence and switching) is being designed and utilized, a need has arisen for short and intermediate range optical links to allow high speed data transfers within various components of a central office. Currently such links require the same expensive circuits and lasers used in long-range optical links. What is needed is a short- and intermediate-range high data rate optical communication system that does not require the cost and complexity of prior art devices and which preferably can be achieved with a reduced form factor compared with prior art devices.
The present invention provides a method and apparatus for reception of optical communications. The present invention provides an optical receiver that includes a feedback circuit to remove dc-offset, and an electrode structure that facilitates high frequency optical signal detection. The electrode structure is designed for ease of fabrication. These features allow the receiver to receive an optical output beam that can be modulated over a wide range of frequencies, duty cycles and amplitudes with the receiver providing very precise definition of the rising and falling edges of the information carried on that output beam. A novel transmitter for use in conjunction with the receiver is also disclosed. In combination these features result in an optical receiver or receiver/transmitter that may be fabricated with relatively low cost and a reduced form factor when compared with prior art optical receivers or receiver/transmitters.
In an embodiment of the invention the optical receiver includes a photo detector, a first electrode and a second electrode. The photo detector is defined on a multilayer semiconductor structure. The photo detector includes a first electrode formed by at least two substantially concentric conductive rings electrically coupled to one another and to a portion of a first layer of the multilayer semiconductor structure. The second electrode is coupled to a second layer of the multilayer semiconductor structure, for the transfer of a current generated by the photo detector responsive to optical emissions.
In an alternate embodiment of the invention a method for fabricating an optical receiver is disclosed. The method comprises the acts of:
providing a multilayer semiconductor structure;
forming a first electrode on a first layer of the multilayer semiconductor structure; and
masking a second layer of the multilayer semiconductor semiconductor structure to define within a mask at least two substantially concentric rings with a gap in a circumference of each of the at least two substantially concentric rings;
forming an electrically conductive lift off layer on the mask; and
removing the lift off layer as a single sheet to form a second electrode including at least two substantially concentric conductive rings electrically coupled to one another and to a portion of the second layer of the multilayer semiconductor structure.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.